The present invention generally relates to methods for determining values for properties of earth formations. The present invention more particularly relates to signal processing methods for determining values for earth formation properties such as density and Pe (photoelectric factor) from photon energy information obtained by a nuclear logging tool.
Borehole logging instruments utilizing nuclear sources, and gamma ray (photon) detectors for obtaining indications of the density and P.sub.e of a formation surrounding the borehole are well known in the art. A typical such device comprises a sonde body containing a gamma ray radioisotopic source and two gamma ray detectors such as NaI crystal scintillators which are spaced from the gamma ray source by about six and twenty inches respectively. Details of such a device may be seen with reference to Wahl, et al.: "The Dual Spacing Formation Density Log", Journal of Petroleum Technology; December 1964 pp. 1411-1416, and with reference to U.S. Pat. Nos. 3,321,625 to Wahl, 3,864,569 to Tittman, and 4,048,495 to Ellis. In the devices described in the article and patents, it is disclosed that the count rate of the far detector is an exponential function of the formation density. However, because the count rate of the far detector is also affected by the mudcake in the borehole, and hole rugosity, the second detector (i.e. the near detector) is used as a means for obtaining information which can be used to compensate the far detector for mudcake and rugosity effects. The method of compensation is set forth in the above-recited Wahl article and is known as the "spine-and-ribs" method. The spine-and-ribs method plots the short spacing detector counting rate for a particular energy window against the long spacing detector counting rate for a different energy window. For a given tool with given spacings, a "spine" is developed in laboratory testing which reflects the relative detector responses for variations in formation density only. The "ribs" extend from the spine and reflect the effect of mudcake thickness and mudcake density on the readings. Thus, by using the count rates of the detectors as inputs, a cross-plot location corresponding to a point on a rib extending from the spine is found. The formation density is then determined by tracing the rib on which the point is located back to the spine, with the intersection of the rib and spine dictating the formation density, and the location on the rib dictating mudcake parameters. Additional advantageous results are obtained, as disclosed in the Ellis patent, by comparing the count rates obtained in different energy windows.
While the spine-and-ribs analysis and improvements thereon for determining formation density is quite effective for the tool disclosed in the above-referenced patents, other tools using nuclear sources and photon detectors exist where such an analysis may not be optimal. In particular, in copending U.S. Ser. No. 07/281,577, a tool is disclosed having a NaI or GSO gamma ray detector preferably placed within one inch of the gamma ray source, and a second such gamma ray detector preferably placed more than four inches away from the source. As described in the copending application, by placing the short spacing detector so close to the source, it will have a non-negative response to an increase of density in the earth formation, while the second detector which is spaced further from the source will have a negative response to an increase in formation density. While the spine-and-ribs approach could be modified by one skilled in the art to provide results for such a tool, it is believed that the different physics associated with such a tool requires different analysis for optimal results. Also, it is believed that because the tool of U.S. Ser. No. 07/281,577 has such a detector which is spaced so close to the source, that fine resolution measurements of earth formation properties should be obtainable. However, data processing techniques to provide fine resolution measurements have not been provided by the art, as the prior art gamma-gamma type tools had detectors which were spaced much farther from the source, and the measurements obtained were recognized as averages over long spacings (approximately one foot).